This invention relates generally to flash electrically erasable and programmable read only memories (EEPROMs), and, more particularly, to an improvement in the speed of programming this type of memory when its memory cells are individually operated with more than two threshold states (termed "multi-state" or "multi-level" operation) to store more than one bit of data.
Existing commercial flash EEPROM products operate each memory cell with two ranges of threshold voltages, one above and the other below a breakpoint level, thereby defining two programmed states. One bit of data is thus stored in each cell, a 0 when programmed into one state and a 1 when programmed into its other state. A chunk of a given number of bits of data is programmed at one time into an equal number of cells. The state of each cell is monitored during programming so that application of programming voltages stops when the threshold level an individual cell is verified to have moved within the range that represents the value of the bit of data being stored in the cell.
In order to increase the amount of data stored in a flash EEPROM system having a certain number of storage cells, the individual cells are operated with more than two threshold level states. Preferably, two or more bits of data are stored in each cell by operating the individual cells with four or more programmable states. Three threshold breakpoint levels are necessary to define four different threshold states. Such a system is described in U.S. Pat. Nos. 5,043,940 and 5,172,338, which are being incorporated herein by this reference. Since an available operating range of the individual cells is divided into an increased number of states, the range of each state is smaller. This requires more time to program since a rate of programming is reduced in order to avoid overshooting a desired small threshold range. If more than four programmable states are provided in each cell, the individual ranges become even smaller, and programming requires even more time.
In order to assure that the state of a cell programmed into a particular threshold range is accurately read after a period of time, whether being operated in two states or multi-states, such programming is usually performed with an added margin. That is, rather than programming a cell into a desired threshold range barely past its threshold breakpoint level, the cell is programmed an amount beyond the breakpoint level by some margin. Use of this margining technique can also increase the amount of programming time since the sizes of the individual threshold ranges are effectively reduced.
Therefore, it is a primary object of the present invention to provide a technique for rapidly programming flash EEPROM systems without having to sacrifice the density or accuracy of the data stored therein.